Oral contraceptives (OCs) are the most popular reversible method of contraception in the US. Although most women using OCs experience reliable protection against pregnancy, nearly half of the 3 million unintended pregnancies in the US annually occur among the 90% of women who use contraception. While lack of adherence to OCs has been assumed to be the primary cause of OC failure, recent findings suggest that obesity may reduce the biological effectiveness of OCs. Prior studies, while provocative, relied upon self- reported height and weight, and were limited in their ability to control for important confounding factors such as adherence to an optimal OC regimen and frequency of sexual intercourse. Also, data on patterns of OC use and whether demographic, lifestyle, or reproductive characteristics are associated with OC discontinuation are limited. These patterns of use are likely to be important to future studies of unintended pregnancy. We propose a 2-year, prospective cohort pilot study, the Fertility and Oral Contraceptive Use Study (FOCUS), with the overall goal of examining methodological issues related to the successful implementation of a large, prospective cohort study to investigate the role of obesity in OC failure. Participants will be 200 OC users attending clinics in the Charlotte, NC area for routine check-ups. During the baseline interview, trained research assistants will collect information on demographic, lifestyle, and reproductive characteristics via questionnaire as well as obtain height, weight, waist circumference, and hip circumference measurements. Upon completion of the baseline interviews, women will be provided diary cards and asked to complete daily entries on OC adherence and frequency of sexual intercourse for a total of 3 months. Every 3 months over a 15-month period, women will be contacted and asked to update information on important factors of interest including adherence to their OC regimen and frequency of sexual intercourse. The 12-month follow-up interview will be conducted in-person in order to obtain updated anthropometric measurements. The remaining follow-up interviews will be via a questionnaire administered either by telephone, mail, or the web. The proposed pilot study will extend prior research in this area by: 1.) obtaining detailed information on patterns of OC use, 2.) investigating factors related to OC adherence and frequency of sexual intercourse, 3.) assessing the validity of retrospective recall of OC adherence and frequency of sexual intercourse, and 4.) collecting regular anthropometric measurements. The exploration of these methodological issues related to recruitment, retention, and data collection will provide critical pilot data for a future R01 grant application to follow a larger group of OC users for an extended period of time. Ultimately, greater understanding of the role of obesity in OC failure will aid public health professionals and healthcare providers in educating and counseling women on effective methods of contraception to prevent unintended pregnancy. PUBLIC HEALTH RELEVANCE: Though the majority of women who use oral contraceptives (OCs) will experience reliable protection against pregnancy, this is not true for all women. Better understanding of factors related to adherence to an optimal OC regimen and frequency of sexual intercourse, as well as patterns of OC use, may aid public health and healthcare professionals in educating and counseling women on using effective methods of contraception to prevent unintended pregnancy.